Intervertebral prosthesis

ABSTRACT

An intervertebral prosthesis for insertion between adjacent vertebrae includes upper and lower prosthesis plates locatable against respective vertebrae and having opposing, concavely curved recesses therein, and a core located between the plates. The core has opposed, convexly curved surfaces received in the recesses of the plates to allow the plates to slide in articulated manner over the core. The opposed surfaces of the core and the recesses of the plates have cooperating spherical curvatures. The recess of each plate surrounds a locating peg projecting centrally from the base of the recess and is bounded by an annular rim, such that the annular rims of the plates are arranged to contact one another at a predetermined limit of sliding movement of the plates over the core. The peg locates loosely in an opening located centrally in a curved surface of the core, whereby the plates can slide over the core in all directions while the peg holds the core captive.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/468,618 filed Aug. 30, 2006; which application is a continuationof U.S. patent application Ser. No. 11/084,224 filed Mar. 18, 2005;which application is a continuation-in-part of PCT/IB2003/001529 filedApr. 24, 2003, designating the United States; which claimed priorityfrom South African Patent Application No. 2002/7517 filed Sep. 19, 2002;the full disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an intervertebral prosthesis.

In the event of damage to a spinal disc it is known practice to implantan intervertebral prosthesis surgically to replace the damaged organ.Several types of prosthesis for this purpose are known and in commonuse.

One type of known intervertebral prosthesis is sold by Waldemar LinkGmbH & Co under the trademark LINK® SB Charité. This prosthesis includesupper and lower prosthesis plates which locate against and engage theadjacent vertebral bodies, and a low friction core between the plates.The core has upper and lower convexly curved surfaces and the plateshave corresponding, concavely curved recesses which cooperate with thecurved surfaces of the core. This allows the plates to slide over thecore to allow required spinal movements to take place. The curvedrecesses in the plates are surrounded by annular ridges which locate, atthe limit of sliding movement of the plates over the core, in opposingperipheral channels surrounding the curved surfaces of the core.

This type of configuration is also described in European PatentPublication Nos. EP 0 560 140 and EP 0 560 141 (both Waldemar Link GmbH& Co). However a drawback of such configurations is that the provisionof the peripheral ribs and channels limits the areas available forbearing and sliding contact between the plates and core, and accordinglythe loads which can be transmitted by the prosthesis. As a result of therelatively small bearing areas, it is believed that at least the corewill be subject to rapid wear and have a relatively short life-span.

EP 0 560 141 also describes one alternative arrangement in which thecurved surfaces of the core carry opposing, elongate keys that locate inelongate grooves in the plates and another alternative arrangement inwhich the plates have opposing elongate keys that locate in elongategrooves in the opposite curved surfaces of the core. These key andgroove arrangements allow the plates to slide freely over the core,within the limits of the length of the grooves, in one plane only.Although allowance is made for some lateral play of the keys in thegrooves, very little sliding movement of the plates over the core cantake place in the orthogonal vertical plane, and this is considered tobe a serious drawback of this design.

2. Description of the Background Art

European Patent Publication Nos. EP 0 560 140 and EP 0 560 141 have beendiscussed above. U.S. Patent Publication Nos. US2002/0035400 andUS2002/0128715 describe intervertebral prostheses with center-postsreceived in passages in a core. The core possesses an annular flangestructure engaged by extensions on the plates.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided an intervertebralprosthesis for insertion between adjacent vertebrae, the prosthesiscomprising upper and lower prosthesis plates locatable against therespective vertebrae and having opposing, concavely curved recessestherein and a core located between the plates, the core having opposed,convexly curved surfaces received in the recesses of the plates to allowthe plates to slide in articulated manner over the core, characterizedin that:

-   -   the opposed surfaces of the core and the recesses of the plates        have cooperating spherical curvatures,    -   the recess of a plate surrounds a locating peg projecting        centrally from the base of the recess, and    -   the peg locates loosely in an opening located centrally in a        curved surface of the core, whereby the plates can slide over        the core in all directions while the peg holds the core captive.

In some embodiments, only one of the plates, typically the operativelylower plate, includes a peg. In other embodiments, both plates includepegs, the pegs opposing one another and locating loosely in respectiveopenings located centrally in the opposed curved surfaces of the core.In each embodiment it is preferred that each peg and the opening inwhich it locates are conical in shape.

According to another aspect of the invention there is provided aprosthesis plate for use in a prosthesis as summarized above, theprosthesis plate having a coarse surface locatable against a vertebra,an oppositely facing concavely curved surface which is complementary inshape to a convexly curved surface of a core of the prosthesis,characterized in that the concavely curved surface of the plate has aspherical curvature and in that the plate includes a conical locatingpeg extending centrally from the concavely curved surface, the peg beinglocatable loosely in a central opening in the convexly curved surface ofthe core.

According to yet another aspect of the invention there is provided acore for use in the prosthesis, the core comprising a one piece plasticsbody having operatively upper and lower curved surfaces, characterizedin that the curved surfaces of the body are spherically curved and atleast one of the surfaces has a central, conical opening thereindimensioned to receive a conical locating peg of a prosthesis plateloosely.

Other features of the invention are set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a cross-sectional view of a prosthesis plate used in aprosthesis according to the invention;

FIG. 2 shows a plan view of the prosthesis plate seen in FIG. 1;

FIG. 3 shows a cross-sectional view of a core used in a prosthesisaccording to the invention;

FIG. 4 shows a plan view of the core seen in FIG. 3;

FIG. 5 shows a cross-sectional view of an intervertebral prosthesisaccording to a first embodiment of the invention in a neutral conditionbefore articulation has taken place;

FIG. 6 shows the prosthesis of FIG. 5 articulated to a limit position;and

FIG. 7 shows a cross-sectional view of an intervertebral prosthesisaccording to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The prosthesis plate 10 seen in FIGS. 1 and 2 may be formed fromtitanium, typically being formed in one piece of grade 5 titanium. Othersuitable materials include titanium nitrides, cobalt chrome alloys, andceramics. The prosthesis plate includes a major surface 12 which ismachined to a coarse, serrated finish 14. Projecting from the surface 12is an elongate fin 16 pierced by transverse holes 18. The oppositesurface of the plate 10 is formed with a recess 20 surrounding aprojecting conical peg 22 of round cross-section at the centre of therecess. The recess is bounded by an annular rim 24. The surface 26 ofthe recess is concavely, spherically curved and has a titanium nitridefinish.

A first embodiment of a prosthesis of the invention, indicated generallyby the numeral 28 in FIGS. 5 and 6, includes a pair of identicalprosthesis plates 10 arranged in opposition to one another; one or bothcan include the concavely curved surface described herein. Theprosthesis 28 also includes a core 30 illustrated in FIGS. 3 and 4. Thecore is made in one piece of a low friction plastics material, in thiscase a polyethylene known as Orthosol, but a variety of other highmolecular weight polyethylenes might also find use. Alternatively, thecore could be composed of a metal, such as cobalt chrome, or of aceramic. The core is generally in the form of a circular,doughnut-shaped toroid and has identical upper and lower surfaces 32which are convexly, spherically curved. The upper and lower surfaces 32are formed centrally with conical openings 34, in this case forming theopposite ends of a circular cross-section passage 36 passing axially andcentrally through the body of the core.

The core surfaces 32 and recess surface 26 have the same radius ofcurvature which is, in this case, 18 mm.

In the assembled prosthesis 28, the plates 10 are arranged in oppositionto one another with the core 30 located between them. The pegs 22 of theplates locate in the openings 34, i.e. in the ends of the passage 36.The combined length of the pegs is less than the length of the passage36, so the inner ends of the pegs are spaced apart from one another by ashort distance 38, as shown in FIG. 5. The transverse dimension of eachpeg is, at all points along the length of the peg, considerably lessthan the diameter of the passage 36 at the corresponding point along thelength of the passage.

The cooperating spherical surfaces 26 and 32 and the relative dimensionsof the pegs 22 and the passage 36 allow the plates 10 to slide orarticulate over the core through a fairly large range of angles and inall directions or degrees of freedom, including rotation about thecentral axis.

At a predetermined limit of articulated movement of the plates relativeto the core, the rims 24 of the plates contact with one another asindicated by the numeral 40 in FIG. 6. Further articulation of theprosthesis 28 beyond this point is not possible. At the limit ofarticulation, the pegs 22 also come into abutment with the side of thepassage 36, as also illustrated in FIG. 6.

It will also be noted in FIG. 6 that the openings 34 at the ends of thepassage 36 are defined by similar cone angles to the pegs 22, so thatcontact between the pegs and the sides of the openings takes placecomplementally over virtually the entire length of each peg.

Throughout the range of possible articulation, the pegs 22 remain in thepassage 36 and prevent the core from separating laterally from theplates 10. In other words, the core is held captive by the pegs duringall possible articulations which can take place. In the illustratedembodiment, the plates 10 are limited to 12° of articulation before therims 24 abut one another, i.e. 12° is the maximum articulation which cantake place.

In the prosthesis 28 described above, the pegs 22 locate in a passage 32which passes right through the core 30. It will however be understoodthat in other embodiments, the pegs could merely locate in blindrecesses or openings in the opposite surfaces of the core without suchopenings actually being joined to one another to form a continuouspassage.

FIG. 7 illustrates a prosthesis according to a second embodiment of theinvention in a view similar to that of FIG. 5. In FIG. 7, componentscorresponding to components illustrated in the earlier Figures areindicated by the same reference numerals.

The lower prosthesis plate 10 in FIG. 7 is identical to the prosthesisplates of the first embodiment. However, the upper prosthesis plate,designated 10.1, has no central peg. Instead, the surface 26 iscontinuously, spherically curved. In all other respects the plate 10.1is identical to the plate 10.

In the assembled prosthesis 50, the core is held captive by the actionof the single peg 22 carried by the lower prosthesis plate 10. The core30 is identical to the core described previously but once again it willbe appreciated that the core could have a blind recess in its downwardlyfacing, curved surface 32 only to receive the single peg 22.

The prosthesis 28, 50 is surgically implanted between adjacent spinalvertebrae in place of a damaged disc. Those skilled in the art willunderstand that the adjacent vertebrae are forcibly separated from oneanother to provide the necessary space for insertion. The plates 10,10.1 are slipped laterally into place between the vertebrae with theirfins 16 entering slots cut in the opposing vertebral surfaces to receivethem.

After insertion of the core between the opposing plates, the vertebraare allowed to move together to hold the assembled prosthesis in place.

The surfaces 12 of the plates 10, 10.1 locate against the opposingvertebrae and, with passage of time, firm connection between the platesand the vertebrae will be achieved as bone tissue grows over theserrated finish. Bone tissue growth will also take place about the fins16 and through the holes 18 therein, further enhancing the connectionwhich is achieved.

Referring to FIGS. 3 and 4, the core 30 used in the embodimentsdescribed above is formed with narrow, angularly spaced, blind passageswhich accommodate titanium pins 42. The core itself is transparent toX-radiation and so would normally be invisible in a post-operative X-rayexamination. The pins 42 serve as radiographic markers and enable theposition of the core 30 to be ascertained during such examination.

Annular grooves 44 are provided in the plates 10, 10.1 to facilitateholding and manipulation of the prosthesis by appropriate instrumentsduring placement into the intervertebral disc space.

Compared to known prostheses, the prostheses 28, 50 described above havea number of advantages, as follows:

-   -   1. The peg or pegs 22 hold the core captive and prevent it from        slipping out sideways.    -   2. At a predetermined maximum limit of articulation, the rims 24        contact one another to prevent further articulation. At the same        time, the peg or pegs 22 contact the sides of the openings 34,        also preventing further articulation.    -   3. An acceptable bearing area of the plates on the core is        obtained with the central peg/central opening configurations        described above. This is particularly advantageous compared to        prior art prostheses where the core has peripheral channels on        its upper and lower surfaces to receive peripheral rims on the        plates at the limit of articulation.

In practice, it may happen that there is imperfect alignment of theprosthesis plates. In the case of very poor alignment, the dual pegconfiguration of FIGS. 5 and 6 gives rise to the possibility that thepegs 22 are laterally offset from one another by a substantial distance.This can in turn lead to the undesirable consequence that the range ofarticulation which can be accommodated is unduly limited, and attemptsto articulate past the limit could result in damage to the core by themisaligned pegs. In the embodiment of FIG. 7 the single peg can bereceived centrally by the core, thereby avoiding this potential problem.

The principles of the invention are applicable to prosthetic discimplants for lumbar vertebrae as well as cervical vertebrae. In thelatter case, the fins 16 will typically be omitted.

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. Therefore, the scope of the invention is limitedsolely by the claims and their full range of equivalents.

1. A method of inserting an intervertebral prosthesis in anintervertebral space, the method comprising: providing an articulatingintervertebral prosthesis including a prosthetic plate comprising twoside surfaces, an outer peripheral plate edge between the two sidesurfaces, and an annular rim, the annular rim having an outer peripheralrim edge extending in an axial direction from one of the two sidesurfaces of the plate, wherein the rim edge is disposed radially inwardof the plate edge; holding the prosthesis with a groove formed in theannular rim of the prosthetic plate; and manipulating the prosthesiswith an instrument to place the prosthesis in the intervertebral space;wherein the groove facilitates manipulation and holding of theprosthesis while the prosthesis is placed in the intervertebral spaceand wherein the annular rim is disposed radially inward of the plateedge around the entire annular rim.
 2. The method of claim 1 wherein theannular rim with the groove bounds a cooperating surface of theprosthetic plate.
 3. The method of claim 1 wherein the groove is formedperipherally on the rim and the prosthesis is held with the peripheralgroove formed peripherally on the rim.
 4. The method of claim 1 whereinproviding the prosthesis comprises providing a second prosthetic plateand the first prosthetic plate is arranged so that the annular rimextends from the plate toward the second plate.
 5. The method of claim 1wherein the prosthesis is held with another groove formed in an annularrim of an opposing prosthetic plate and the grooves facilitatemanipulation and holding of the prosthesis while the prosthesis isplaced in the intervertebral space.
 6. The method of claim 5 wherein theannular rim of the opposing plate bounds a cooperating surface of theopposing prosthetic plate.
 7. The method of claim 1 wherein the grooveis an annular groove.
 8. A method of inserting an intervertebralprosthesis in an intervertebral space, the method comprising: providingan intervertebral prosthesis including first and second prosthesisplates, each plate having an inwardly and an outwardly facing surfaceand an outer peripheral edge between the inwardly and outwardly facingsurface, each plate comprising an annular rim extending in an axialdirection from the inwardly facing surface and having a rim edgedisposed radially inwardly of the outer peripheral edge and a grooveformed in the rim edge, the prosthesis further comprising a corepositioned between the plates and movable with respect to the plates;holding the prosthesis plates with the grooves; and manipulating theprosthesis to place the prosthesis in the intervertebral space; whereinthe grooves facilitate manipulation and holding of the prosthesis by theinstrument while the prosthesis is placed in the intervertebral spaceand wherein the annular rim is disposed radially inward of the plateedge around the entire annular rim.
 9. The method of claim 8, whereinthe annular rims with the grooves bound cooperating surfaces of theprosthetic plates.
 10. The method of claim 8, wherein the grooves areformed peripherally on the rims and the prosthesis is held with theperipheral grooves formed peripherally on the rims.
 11. The method ofclaim 8, wherein the annular rims extends from each of the plates towardthe other opposing plate.
 12. The method of claim 8, wherein theprosthesis is held with another groove formed in an annular rim of anopposing prosthetic plate and the grooves facilitate manipulation andholding of the prosthesis while the prosthesis is placed in theintervertebral space.
 13. The method of claim 8, wherein the grooves areannular grooves.